Lithium manganese oxide spinel is a potential candidate for lithium-ion battery cathodes due to its 3D network of lithium pathways within the structure, low toxicity, reasonable capacity, and low cost. However, this spinel suffers from capacity fading due to fracturing of the structure either through the Jahn-Teller distortion, alternative phase formation via non-stoichiometry, or loss of crystallinity. Historically, cycle life enhancements have been achieved through stabilization of the spinel structure via transition metal doping on the B-site of the lattice. Iron as a dopant is of interest due partly to its low toxicity but primarily its low cost. By adding the redox couple of this transition metal, this spinel can satisfy higher voltage (5.0 V) applications should compatible electrolytes be produced.
While B-site doping has proven effective, this effort proposes to observe if cycle life can be even further enhanced in LixMn2-yFeyO4 (LMFO) by also substituting anion dopants. Previously our group has incorporated chlorine and fluorine into the base LiMn2O4 material using solid state processing methods and demonstrated cyclability to 32 cycles as a result of weakening the Li-O bond for easier extraction from the lattice. Others have also added chlorine and more commonly fluorine and demonstrated performance enhancements. However rather than incorporating the halide into the lattice, most studies focus on surface modification of the spinel. The present study intends to look more into the cycle life benefits of the less-studied chlorine incorporation into the bulk of an iron-doped spinel via in situ combustion synthesis of the material.
A glycine nitrate combustion method was used to synthesize iron doped chlorinated lithium manganese oxide spinel (LMFO-Cl). Stoichiometric amounts of Li(NO3), Mn(NO3)2∙4H2O, FeCl3, and NH2CH2COOH (glycine, Alfa Aesar) in a 1:1 metal to glycine ratio were dissolved into the aqueous solution. The solution was heated to evaporate the water and form a gel, which was heated further to 250 °C when auto ignition occurs. The resultant ash was fired at 600 °C for two to six hours to achieve the desired phase. Material characterization was performed using X-ray diffraction, X-ray fluorescence, BET surface area analysis, thermogravimetric analysis, and scanning electron microscopy. Cathode materials were mixed with carbon black and Teflon in a ratio of 85:10:5 of active to carbon to Teflon binder and calendared to 0.04 cm, punched into disks and pressed onto aluminum mesh opposite a Li metal anode. 2025 button cells were filled with 1 molar LiPF6 in a proportional mixture of diethyl carbonate, dimethyl carbonate, and ethylene carbonate. Electrochemical characterization was performed using electrochemical impedance spectroscopy (Solartron 1260) and cycling on an ARBIN MSTAT4 battery cycler system.
Results and Conclusion:
A nanoscale LixMn2-yFeyO4-zClz material was synthesized in three different compositions of y and z and desirable phase and morphological properties were achieved. The x-ray diffraction pattern indicated no presence of impurities while the x-ray fluorescence showed maintenance of the chlorine within the material after calcination. BET surface area analysis determined the surface area to be 8.48 m2/g with a highly desirable large pore size for ion transfer (14nm). The material was demonstrated to repeatedly achieve minimal capacity loss with short term cycling in all compositions and yielded 98% of the original discharge capacity at the 250th cycle for the case where y = 0.195 and z = 0.028. Figure 1 shows the cycling behavior out to 250 cycles demonstrating the capacity retention of this composition, LixMn1.805Fe0.195O3.972Cl0.028. Discharging below the 4.1 V plateau to 2.25 V did not immediately impact the cells as the reaction was reversible for all three compositions synthesized in this study. The primary limitations for these experimental cells were the inadequacy of the electrolyte preventing access to the higher voltage plateau at 5.0 V where more capacity could be achieved. Differential capacity curves across the 300 cycles (not shown here) demonstrate typical electrolyte degradation behavior. However the cathode chemistry appears to withstand advanced cycling and further work with electrolyte improvements could emphasize the extent of its performance capabilities.